Volume 34, Number 6, June 1999

Maximizing Ceramic Furnace Roll Performance

Ceramic roller properties and dimensional specifications are critical to the glass
tempering process. While the manufactured quality of the roller is essential, the
environment in which the rollers operate and the processes and procedures practiced before
and during tempering directly influence the life and performance of the rollers. The
combined influences of the roller quality, environmental effects on the rollers and the
roller-to-glass interface, along with the methods and frequency of cleaning the rollers,
directly relate to the life and cost effectiveness of the rollers. These influences
ultimately affect the productivity and profitability of the glass tempering operation.
Additionally, the scientific identification of microscopic defects in tempered glass is
essential to isolating the cause and resolution of such defects.

Maximizing ceramic furnace roll performance requires an understanding of the total
tempering system. This article addresses the critical properties and specifications
required of fused silica ceramic rollers and the influences of the glass tempering plant
environment on the rollers. Roller maintenance and cleaning methods are discussed along
with methods for identifying microscopic defects in glass using optical microscopes,
scanning electron microscopy and energy dispersive spectroscopy.

A History of Ceramic Furnace Rollers

The first fused silica rollers were introduced to horizontal tempering furnaces in
1972. Since their introduction, Vesuvius Zyarock fused silica roller applications
have expanded into float glass annealing lehr applications. Roller manufacturing and size
capabilities have increased from the original 1.25 inches in diameter to over 12 inches
today. Length capabilities have also increased from 45 inches in 1972 to over 246 inches
today. Zyarock rollers have effectively replaced asbestos and steel rollers in
horizontal glass tempering furnaces.

Material

Fused silica is uniquely qualified for tempering roller applications due to its
physical and chemical properties. It has a non-crystalline, amorphous structure with very
low thermal expansion, low thermal conductivity and a very high resistance to thermal
shock. It is compositionally a gas (SiO2) which is inert and resistant to corrosion,
making it compatible with tempered glass.

Manufacturing

There are many critical steps in the manufacturing process that can and will affect the
performance of the fused silica tempering rollers. The process requires tight controls
from the raw materials through the casting and firing processes to produce a homogenous
and thermally stable blank.

The blank quality will directly affect the surface finish, strength and the hot Total
Indicated Runout (TIR) of the finished roller. TIR is critical, especially when the
rollers are at operating temperatures. An inferior roller blank may have a higher thermal
expansion resulting in bow or warp at tempering temperatures. The blanks are machined with
diamond tooling to achieve the tight tolerances of the finished roller.

Surface finishes are critical to the roller-to-glass interface and affect the heat
transfer from the rollers to the glass. Rollers may be polished, textured or spiral
machined. The surface finish specification will vary with each furnace manufacturer. It is
important that replacement rollers meet OEM specifications.

End Caps

A variety of end cap designs are available depending upon the application temperature,
furnace design and drive system. These include popular RTV silicon-bonded caps, custom
designs, patented high temperature caps for use up to 138°F, and for certain furnace
designs and rollers without end caps. The end caps should not induce mechanical stress on
the ceramic. They must be concentric to the roller to assure good TIR and the attachment
method must be carefully considered.

Quality

We recommend all specifications be confirmed in writing and preferably with a detailed
drawing. Certified quality inspection reports for each roller are available. It is
suggested that incoming inspection be conducted on rollers if they are not purchased from
an ISO certified supplier. All of our manufacturing locations are certified to ISO 9001.

Environmental Concerns

The manufacturing environment and production practices influence the tempering
operation, specifically the roller life and performance. For this reason, plant
cleanliness is of the highest importance. Dirt and dust on the glass or that passes into
the furnace cavity will deposit on the rollers. This debris can result in bottom surface
mechanical damage or be picked up and re-deposited on the glass.

Glass Seaming and Fabricating

While glass seaming and fabricating can be significant sources of dust, the quality of
these operations is also critical. Edge work must always be complete, smooth and free of
chips or small shards protruding from the glass, as they will break away during the heat
treating process. These particles may stick to the rollers and result in bottom surface
defects. When viewed by scanning electron microscopes (SEM), the defects are bottom
surface break outs or chips in the glass and deposits on the glass surface. Always check
the quality of the edge work, seaming and drilled holes and be sure the glass is clean.

Plant Dust Control

Debris and airborne dust in the tempering plant will cause defects in the glass and
limit the effectiveness of the rollers. Debris may be carried into the furnace from
different operations such as silk-screening, painting operations and fabricating. Paint
located on the edge of the glass may drip on the rollers. Airborne paint can deposit on
the rubber rollers in front of the furnace and track into the furnace cavity.
Ceramic-based frit paint bonds to the fused silica rollers and cannot be washed from the
roller surface. Paint defects on glass may appear as white haze or scuff marks while it is
typically a deposit that can be viewed by SEM. Energy dispersive spectroscopy (EDS)
reveals the unique components of the paint including lead and chromium.

Glass Washer

Effective use of the glass washer is critical to every tempering operation. The glass
should be washed just prior to tempering. Washing the glass and staging it for extended
periods of time before tempering is ineffective because dust collecting on the glass will
be carried into the furnace. The glass washer must be operating properly with the
detergents, brushes and rinse water at the manufacturers recommended settings.
Maintenance of the glass washer is equally important to ensure the glass is clean and
without residue or debris that will be carried into the furnace. Certain materials used to
separate the glass are difficult to remove if the washer settings are not correct. These
defects may appear as chips under an optical microscope but under SEM they are deposits.
EDS shows the spectrum of a high carbon component typical of Lucor . Proper settings
and strict adherence to the maintenance schedules of the glass washer will minimize
certain glass defects and protect the rollers from contamination.

Tempering Furnace

Cleanliness continues as the fundamental theme for optimizing the life and performance
of the Zyarock fused silica tempering rollers. Several sources of contamination can
be generated within the furnace. Glass tempering furnace operators are encouraged to
consult with the furnace manufacturer for maintenance schedules and the proper methods for
handling refractory insulation, roll seals, heating elements and sulfur dioxide.

Fiber Insulation

This is one of the most common and prolific sources for glass defects. The furnace
cavity must be kept free of refractory fiber dust. This dust is often created when furnace
maintenance is performed, although it may also develop as the fiber degrades with time and
temperature. The detrimental effects of refractory fiber contamination are compounded in
the presence of aspiration and forced air convection, which stirs up the dust.

Roll Seals

The roll seals are important to maintain heat balance in the furnace, energy
conservation and to protect the roller end caps, furnace drive and bearings. Many roll
seals are manufactured with refractory fiber insulation that is surface hardened. The roll
seals may produce dust if they are broken, damaged or worn. Any dust should be vacuumed
away and broken roll seals replaced immediately. Plugging gaps or holes in the roll seals
with bulk fiber insulation is not recommended, as it may introduce dust into the furnace
cavity. Alternative seal materials are available.

Heating Elements

Maintaining the heating elements and other metallic components, and following the OEM
furnace builders specifications for replacements, may prevent metallic contamination
in the furnace cavity and on the rollers. Certain metal alloys or failure to properly
install or maintain the heating elements may result in metal contamination on the rollers
and on the glass.

Sulfur Dioxide

Sulfur dioxide is useful as a dry lubricant to form a coating on the glass and a
barrier between the glass and the rollers. Debris on the roller surface will often leave
an impression or defect in the glass surface. Sulfur Dioxide provides the barrier to
prevent or minimize defects, but it will react with the sodium ions in the glass at
elevated temperatures to form sodium sulfate. Excessive use of sulfur dioxide leads to the
formation of sodium sulfate deposits or nodules on the rollers. While these deposits are
water-soluble, the nodules may pull out grains of fused silica from the roll surface
leaving pits in the roller. The pits have more surface area and become the sites for more
build-up on the roller surface. Sodium sulfate nodules can transfer from the rollers to
the glass surface resulting in defects. Limited use of sulfur dioxide or dosing the gas
often reduces the frequency and concentration of sodium sulfate nodules.

Roller Cleaning and Maintenance

The ceramic rollers should only be washed with water. Never use detergents! Detergents
will react chemically with the rollers, resulting in denitrification and roller failure.
Due to the extremely low thermal expansion, the rollers can be washed hot. Glass debris on
the roller surface can be shocked off when cold water is applied properly to a hot roller.
Precautions should be taken to prevent the end caps from getting hot during the cleaning
process. Rollers can also be effectively cleaned at room temperature. Lint-free towels and
certain abrasive pads such as 3Ms Scotch Brite are effective. The rollers
should be dried following cleaning. If the rollers are soaked with water and heated too
quickly, steam may develop in the roller and cause it to fail.

Other cleaning methods include the use of tack cloth to remove surface dust. Sandpaper
can be used, but should be avoided if possible. Sanding may induce waves or an
out-of-round condition to the roller surface. On certain rollers, the surface
specification requires a textured finish. Sanding the rollers may remove the finish and
polish the surface. If sanding is required, take great care and be absolutely certain to
avoid introducing dust into the furnace environment.

The frequency of roller cleaning varies greatly and is usually determined by the
cleanliness of the tempering environment. As discussed, the plant environment, furnace
cleanliness, fabricating practices, glass cleanliness, sulfur dioxide usage and other
factors will influence roller cleanliness. There are methods and measures for determining
the cleaning frequency for each furnace.

Refurbishing may also be performed on rollers that have excessive surface build-up and
a moderate amount of surface damage. This usually involves re-machining the rollers and
removing up to .005 inches from the diameter. Before proceeding, consider the TIR of the
rollers, wear and end cap condition. A roller with excessive TIR may look better after
refurbishing, but it may not perform better. It is recommended that rollers be re-machined
only one time to avoid significant diameter changes.

Summary

Maximizing the performance of ceramic furnace rollers requires a total systems
approach. Roller quality and stability are essential as are the cleanliness of the factory
and the furnace, the operating practices and glass cleanliness. Defects in tempered glass
can usually be identified, isolated and resolved through the use of optical microscopy,
scanning electron microscopy and energy dispersive spectroscopy.

The above article was presented at Glass Processing Days 99 in Tampere,
Finland.

Renald D. Bartoe is product line manager, Americas for Vesuvius McDanel; Frederick
Caillaud is Zyarock development manager for Vesuvius France; Dr. John Dodsworth is
technical manager for Vesuvius McDanel; and Jerry Osele is research engineer for Vesuvius
McDanel.